The present invention relates to the stabilization and control of large bodies, and more particularly to the efficient neutralization and damping of externally or internally originating periodic, non-accumulating torques to which such a body may be subjected. Such systems are in wide use today, ranging from control systems for ocean-going ships to stabilization systems for earth-orbiting satellites. Thus, while the present invention will be particularly described with respect to space stations, satellites, etc., it will be understood that the invention has perhaps even greater utility in terrestrial applications.
The successful operation of future space stations, platforms, or vehicles requires that they be capable of controlling and maintaining a desired orbital flight attitude (e.g., a solar inertial attitude). Large structures in low earth orbit, if not uniformly balanced, are particularly susceptible to gravity gradient disturbance torques and to aerodynamic torques on one or more axes. In the general case, the three principal axes of inertia will not be of equal magnitude. A space station orbiting with one of its principle axes in the orbit plane will produce cyclic, nonaccumulating torques about the two out-of-plane axes, and will produce a biased, accumulating torque about the third axis lying in the orbit plane. These constant disturbance torques will act on a structure and cause its total angular momentum to fluctuate accordingly. The major, periodic constituent of this variation may have, in the case of a typical space station configuration, an amplitude in the range of 15,000 to 90,000 ft-lb-s. The magnitude of angular momentum buildup, of course, will depend upon the degree of station imbalance.
If a particular attitude, such as a solar inertial attitude, is to be maintained, appropriate torques must be applied about each axis to null the gravitationally and aerodynamically induced torques. Unlike the cyclic disturbances, any biased torques that exist will produce an angular momentum buildup necessitating repeated desaturation (such as with attitude control jets, magnetic torquers, or a similar system).
Angular momentum management and attitude control of a small space vehicles has been reliably provided by control moment gyros and reaction wheels. These devices operate on the gyroscopic principles of spinning rotors, and are capable of producing control torques and storing angular momentum. A reaction wheel produces torque and stores angular momentum by commanding a change in rotor speed about a fixed spin axis. Control moment gyros and double gimbaled control moment gyros operate by attempting to rotate a constant speed rotor about a given axis, resulting in a torque which is perpendicular to both axes. When momentum saturation occurs, no further torquing may be obtained in the direction of saturation. In general, both these devices have storage capabilities of about 6.0 ft-lb-s/lb of flywheel, and operational energy requirements in the neighborhood of 50 watts/1000 ft-lb-s. Advancements in material technology may produce composite flywheels with slightly greater capabilities, but unfortunately, problems with creep and material integrity reduce the advantages of composites.
Other system problems are of greater concern, however. A space station class vehicle, ocean-going ship, etc., will have large momentum control requirements necessitating the use of many control moment gyros or reaction wheels. Each unit will ordinarily require individual control and will occupy useful structure area which might otherwise be used for more constructive purposes. Furthermore, a nominal system reliability will require either a very high unit reliability or the installation of additional backup units.
Of perhaps even greater concern are the safety issues concerning rotor rupture confinement. High speed rupture of these devices could cause substantial damage to the surrounding structure, and jeopardize crew safety. Stringent safety requirements can therefore be expected to cause system weights to increase substantially.
For small to moderate torquing requirements, electromagnetic torquebars have been successfully used for complete, three axis angular momentum control. A torquebar's energy efficiency and system weight, however, severely restrict its use for all by small amplitude desaturation torquing.
To summarize, control moment gyros, magnetic torquers, and reaction wheels exhibit poor energy efficiencies and, when reliability and safety issues are properly addressed, inflict large weight penalties.
A need therefore remains for an efficient, light weight, reliable, and versatile momentum control method and apparatus which can better meet the cyclic and periodic torquing and momentum compensation and management needs of an extended body, such as a space station, ocean-going ship, and so forth. The method and apparatus should preferably use less energy and be lighter in weight than prior art devices.